Showing papers in "Magnetic Resonance in Chemistry in 1993"

Gradient selection in inverse heteronuclear correlation spectroscopy

Abstract: After a brief theoretical description, new gradient-selected, proton-detected heteronuclear correlation sequences are introduced. The gs-HMBC and gs-Relayed-HMQC are closely related to the original gs-HMQC proposed by Hurd and John. A new approach to obtain pure absorption line shapes in gradient selected spectroscopy is used to measure phase-sensitive gs-HMQC spectra, to carry out multiplicity editing in HSQC spectra and to distinguish direct and long-range correlations in HMQC/HSQC–TOCSY spectra.

Carbon‐13 chemical shift anisotropies of solid amino acids

Abstract: An approach to spinning sideband (SSB) separation of protonated and non-protonated carbons in slow spinning conditions via dipolar dephasing is proposed. This method offers two features: SSB spectral simplification, which greatly reduces the overlaps encountered in a usual SSB spectrum, and spectral distinction of overlapped resonances for protonated and non-protonated carbons. The 13 C chemical shift anisotropies of the 20 amino acids as polycrystalline powdered solids were completely measured with the SSB intensity analyses as an application of the suggested approach

Water exchange on [Gd(H2O)8]3+ and [Gd(PDTA)(H2O)2]‐ in aqueous solution: A variable‐pressure, ‐temperature and ‐magnetic field 17O NMR study

Abstract: 17O NMR longitudinal and tranverse relaxation rates and chemical shifts were measured at variable temperature at three magnetic fields (1.4, 4.7 and 9.4 T) for aqueous solutions of the complexes [Gd(H2O)8]3+ and [Gd(PDTA)(H2O)2]−. The transverse relaxation rates and chemical shifts were analysed in the light of recent EPR line width measurements to obtain the parameters for water exchange kinetics: k = (8.30 ± 0.95) × 108 and (1.02 ± 0.10) × 108 s−1, ΔH‡ = 14.9 ± 1.3 and 11.0 ± 1.4 kJ mol−1 and ΔS‡ = −24.1 ± 4.1 and −54.6 ± 4.6 J K−1 mol−1 for [Gd(H2O)8]3+ and [Gd(PDTA)(H2O)2]−, respectively. The longitudinal relaxation rates were used to obtain the parameters for the rotation time of the complexes: τ = (2.9 ± 0.2) × 10−11 and (7.9 ± 0.3) × 10−11 s and Ec = 15.1 ± 1.5 and 19.2 ± 1.1 kJ mol−1 for [Gd(H2O)8]3+ and [Gd(PDTA)(H2O)2]−, respectively. The 17O NMR transverse relaxation rates were measured at variable pressure, and were used to determine the activation volumes for the water exchange process: ΔV0‡ = −3.3 ± 0.2 and −1.5 ± 0.5 cm3 mol−1 for [Gd(H2O)8]3 + and [Gd(PDTA)(H2O)2]−, respectively. It can be concluded that water exchange occurs via an associative limiting A mechanism for [Gd(H2O)8]3 + (without excluding Ia) and an associative interchange, Ia, mechanism for [Gd(PDTA)(H2O)2]−. These water exchange kinetic and mechanistic results are compared with those for the heavy Ln3+ aqua ions and the Gd3+ complexes with DTPA5− and DOTA4−.

13C NMR of pyrazoles

Abstract: The carbon-13 chemical shifts and coupling constants of 1168 pyrazoles are reported, some of them in several solvents. These data are briefly discussed on the basis of statistical treatment and qualitative comparisons.

Tautomerism in 2-hydroxy-1-naphthaldehyde schiff bases in solution and the solid state investigated using 13C NMR spectroscopy

Abstract: 13C and 1H NMR spectra of six Schiff bases formed from 2-hydroxy-1-naphthaldehyde and substituted anilines were measured and assigned in a variety of solvents. Based on the 13C chemical shifts and 3J(CH,NH), the ratio of the keto-amine and phenol-imine tautomers was derived using both 13C chemical shifts and 1H−1H coupling constants, the two methods giving comparable results. The solid-state 13C NMR spectra of two of these Schiff bases were also measured and interpreted. Based on chemical shift assignments from solution NMR, which showed a relationship between certain shifts and the proportions of the tautomers, it has been shown that these compounds exist essentially as the keto-amine form in the solid, the tautomer previously shown to be the more thermodynamically stable.

Correlation between 27Al and 71Ga NMR chemical shifts

Abstract: A linear relationship has been found to exist between the aluminium-27 and gallium-71 NMR chemical shifts of a series of structurally analogous aluminium and gallium compounds having only oxygen in the first metal coordination spheres; ∂ 71 Ga=2.83 (δ 27 Al)-4.50. This relationship allows the prediction of 71 Ga chemical shifts for such gallium compounds from the 27 Al values already known for their aluminium analogues, and also a greater understanding of observed 71 Ga chemical shifts

Isogambogic acid and isomorellinol from Garcinia hanburyi

Abstract: Three xanthone derivatives, gambogic acid, isogambogic acid and isomorellinol, were isolated from the dried latex of Garcinia hanburyi. Two of them, isogambogic acid and isomorellinol, are new. Determinations of the structures and stereochemistry were achieved independently by a series of NMR experiments including COSY, ROESY, HMQC, HMBC and selective INEPT. Isolation of isomorellinol from G. hanburyi provided important chemical evidence to link this species to G. morella. The presence of gambogic acid and isogambogic acid, however, demonstrated the difference between these two species. Cytotoxic evaluation of these isolates revealed that all three were active against KB and drug-resistant KB-V1 cell lines.

Substituent effects on deuterium isotope effects on nuclear shielding of intramolecularly hydrogen-bonded aromatic ketones, aldehydes and esters

Abstract: Deuterium isotope effects on 13C nuclear shielding, nΔC(OD), were investigated for a large number of substituted ortho-substituted aromatic hydroxyacyl compounds of the type salicylaldehyde, 2-hydroxyacetophenone, etc. The isotope effects were analysed as a function of substitution patterns. 2ΔC-2(OD) shows a linear dependence on ΔOH, with one slope for 4- and 6-substituted derivatives and another for 5-substituted derivatives. A similar pattern is also found for the long range isotope effects. In addition, isotope effects also correlate with 13C chemical shifts. Isotope effects in o-hydroxyacetophenones can be predicted from the δOH and nΔC(OD) values of salicylaldehyde, and vice versa. The isotope effects were analysed in terms of a vibrational term related to the intramolecular hydrogen bond and an electronic term related to the substituent influence on the transmission of the effect along the pathway between the deuterium-substituted oxygen and the carbon in question. 2ΔC(OD) isotope effects are found to depend on the one-bond bond order of the CC bond of the unsubstituted hydrocarbons. Long-range isotope effects are observed to the olefinic carbons of 2-hydroxychalcone, whereas no effects beyond C-1′ are seen in o-hydroxyketones or o-hydroxybenzophenones. The tautomeric equilibrium of usnic acid is perturbed by the removal of the weak hydrogen bond between 9-OH and the carbonyl group at C-1.

Interconversion of diastereomeric complexes involved in Rh(I)-catalyzed asymmetric hydrogenation: A (31P, 31P) EXSY NMR study

Abstract: Rhodium(I) complexes containing chiral chelating diphosphines, e.g. R,R-dipamp [(R,R)-1,2-bis(o-methoxy-phenylphenylphosphino)ethane] or S,S-chiraphos [(2S,3S)-bis(diphenylphosphino)butane] and a prochiral olefin, e.g. methyl (Z)-α-acetamidocinnamate (mac), form diastereomeric complexes which differ only in which enantiotopic face of the olefin is bound to rhodium and can be observed by 31P NMR. The interconversion of the two diastereomers has been observed with (31P,31P)-{1H} 2D EXSY techniques for various mixing times, Tm, and in three different solvents (CD2Cl2, CD3OD and CD3COCD3). The results clearly demonstrate that both inter- and intramolecular ligand exchange processes are occurring, and that diastereomeric interconversion cannot proceed exclusively by complete mac dissociation followed by readdition to the opposite enantiotopic olefin face. The new results are consistent with previous DANTE NMR experiments. The differing results in CD2Cl2, CD3OD and CD3COCD3 solutions suggest the possibility of solvent involvement in the rate-limiting step of ligand exchange. The 2D EXSY method appears to be the method of choice for elucidating the qualitative exchange pathway of this complex system.

Analysis of long-range through-space couplings via an intramolecular hydrogen bond

Abstract: 2-Fluorobenzamide and its N-methyl derivative show spin-spin couplings between the aromatic fluorine and the nitrogen and carbon of the amide group, which are absent in the corresponding N,N-dimethylamide. The N-methyl compound, in which there is a hydrogen bond between the fluorine and the amide hydrogen, also shows fluorine couplings to the nuclei of the methyl group, and these are diminished for solutions of the amide in polar solvents. Such couplings are absent in the N,N-dimethylamide, which does, however, show fluorine couplings to the proximate methyl group. Calculations at the INDO level employing the IPPP method give reasonable agreement with the magnitudes of the observed 4J(F,N) values in the N-methylamide and show that the Fermi contact contributions are dominant and that the spin information is transmitted mainly through the hydrogen bond.

Determination of chemical‐shift tensor orientations in methylene groups by separated‐local‐field NMR

Abstract: The 13 C chemical-shift tensor orientations in the OCH2CH 2 O moieties of poly(ethylene oxide) (PEO) and poly(ethylene terephtalate) (PET) were determined by means of 1 H- 13 C separated-local-field NMR spectroscopy, and the literature values for the shift tensors in polyethylene (PE) and polyoxymethylene (POM) were improved. An angular resolution of ±4 o was attained on standard equipment with 90 o pulse lengths exceeding 4 μs. Relatively high uniformity of the methylene tensor orientations, complying with local molecular symmetry, was found

1H and 13C NMR assignments with coordination‐induced shift calculations of carbon σ‐bonded ortho‐metalated rhodium(III) and iridium(III) complexes

Abstract: Complete 1H and 13C NMR assignments and coordination-induced shift [CIS, δ(complex) – δ(free ligand)] calculations for carbon σ-bonded ortho-metalated rhodium(III) and iridium(III) complexes are described. NMR parameters are used as probes of van der Waals interactions and ring current anisotropy in the ligand's environmental framework and to monitor σ-donation and π-back-bonding within the ligand in this family of ortho-metalated complexes. CIS measurements indicate large charge redistribution within the ligands as a result of ortho-metalation to the rhodium(III) or iridium(III) cationic metal centers. The pulse sequences, attached proton test (APT), 1H1H homonuclear correlation spectroscopy (COSY), single-frequency off-resonance decoupling (SFORD) and 1H13C heteronuclear correlation spectroscopy (HETCOR), were utilized to assign all proton and carbon resonances of [Rh(ppy)2Cl]2, [Ir(ppy)2Cl]2, [Ir(ptpy)2Cl]2, [Ir(mppy)2Cl]2, [Rh(ppy)2bpy]+, [Ir(ppy)2bpy]+, [Ir(ptpy)2bpy]+ and [Ir(mppy)2bpy]+, where ppy is 2-phenylpyridine, ptpy is 2-(p-tolyl)pyridine, mppy is 3-methyl-2-phenylpyridine and bpy is 2,2′-bipyridine.

Experimental (13C and 15N NMR spectroscopy) and theoretical (6–31G) study of the protonation of N‐methylazoles and N‐methylbenzazoles

Abstract: The 13 C and 11 N NMR spectra of 11 N-methyl derivatives of azoles and benzazoles (imidazole, pyrazole, 1,2,4 triazole, indole, benzimidazole, 2H-inidazole, 1H-indazole, 2H-benzotriazole, 1H-benzotriazole and carbazole) in trifluoroacetic and sulfuric acid are reported. Quaternary salts were used asmodel compound. In the care of azoles (from pyrrole to pentazole) ab initio calculations at the 6-31G level were carried out to determine the protonation site

Solasodine and diosgenin: 1H and 13C assignments by two‐dimensional NMR spectroscopy

Abstract: Solasodine and diosgenin have been widely used as starting materials for thepartial synthesis of cortisone, progresterone and C21 steroidal derivatives (oral contraceptives). This study involved the total 1 H and 13 C spectral assignments of these two important compounds by using two-dimensional NMR techniques (COSY, CSCM and HMBC), and resulted in unambiguous spectral assignments, especially in the convoluted regions of the specha ( 1 H, 0.7-2.0 ppm; 13 C, 28-35 ppm)

Variable‐temperature NMR studies of 2,6‐dihydroxy acylaromatic compounds. Deuterium isotope effects on chemical shifts, isotopic perturbation of equilibrium and barriers to rotation

Abstract: A series of 2,6-dihydroxy acylaromatic compounds were investigated to characterize the rotational and hydrogen bonding properties of the carbonyl group. Deuterium isotope effects on 1H and 13C chemical shifts due to deuteriation of OH groups were determined at both ambient and low temperature. In the latter case isotope effects on chemical shifts of the individual rotamers can be determined. Deuteriation of one of the OH groups may lead to isotopic perturbation of the tautomeric equilibrium of the carbonyl group and the two hydroxyl groups. The perturbation was found to be larger in ketones than in esters. Complete band shape analysis of the OH resonances of the esters and ketones in a temperature interval above and below the coalescence temperature led to ΔG≠, AH≠ and ΔS≠ values for various concentrations of added THF-d8. ΔS≠ was found to be strongly negative. Temperature coefficients for the shift of the OH resonances showed large variations for esters and ketones owing to the different hydrogen bond patterns. The esters have two intramolecular hydrogen bonds, one strong and an additional weaker one between the OH and OR groups. The second OH group of the ketones was shown to point primarily towards C-5. Increasing amounts of THF-d8 increased the amount of this rotamer. The anisotropy of the XCO group at C-2, C-6 was shown to lead to a low-field shift of C-2, very different from that found for CO groups without hydrogen bonds. The anisotropy caused by OH groups can also be estimated. On the basis of the thermodynamic parameters, a model for the rotation of the ester group is suggested. The rate-determining step involves both intramolcular hydrogen bonds, which are twisted out of the ring plane to form hydrogen bonds to the solvent or other hydrogen bond acceptors.

17O NMR spectroscopy: Study of intramolecular hydrogen bonding in phenols and salicylaldehydes

Abstract: Natural abundance 17O NMR data for fifteen 2- and 4-substituted phenols, ten 3-and 5-substituted 2-hydroxybenzaldehydes and eight 3-substituted benzaldehydes, recorded at 75°C in acetonitrile are reported. The chemical shift change due to intramolecular hydrogen bonding for the phenolic oxygen was found to be 10–14 ppm shielding. In acetonitrile, the 17O NMR chemical shift for phenol signals was insensitive to added water up to water concentrations of 0.5 mole fraction. The 17O NMR chemical shifts of the 4-substituted phenols gave an excellent correlation (r = 0.990) with anisole 17O NMR data; the data also correlated moderately well with σ− (r = 0.974). The chemical shifts of the 3-substituted benzaldehydes were correlated with σ+ values (r = 0.991). A plot of the carbonyl chemical shift data for the substituted 2-hydroxybenzaldehydes versus the carbonyl data for 3-substituted benzaldehydes gave a slope of 0.87 and with r = 0.960. The plot of the 4-substituted phenol data with that for OH of the corresponding 2-hydroxybenzaldehydes gave a slope of 1.04 with r = 0.996. Proton to oxygen coupling for the phenolic group of several of the intramolecular hydrogen bonded systems was observed directly [J(OH) = 58–92 Hz]. MM2 and MOPAC calculations predict that the hydrogen bond distances and angles for the substituted 2-hydroxybenzaldehydes and the partial atomic charges for the carbonyl groups (AMI) were essentially constant. After corrections for electronic effects the chemical shift changes due to hydrogen bonding for the donor (ΔδHBD) and acceptor (ΔδHBA) of the carbonyl–phenol intramolecular bonding system were 5–12 and 30 ± 2 ppm, respectively. The ΔδHBA value was between those for keto and ester acceptors consistent with the relative basicity of the aldehyde group. The ΔδHBD value was substantially larger than those for phenolic donors to keto and ester groups.

Determination of the structure of alkoxysilane polymers in solution by natural abundance two dimensional 29Si‐29Si INEPT DQF COSY NMR

Abstract: In order to determine the nature and the structure of polymers formed by condensation of alkoxysilanes, a two-dimensional INEPT double quantum filtered COSY NMR experiment was developed to observe 29 Su- 29 Si correlations at 29 Si natural abundance. This experiment was tested on a mixture containing a trimer, [(CH 3 ) 3 SiO] 2 Si(CH 3 ) 2 , and tetramer, [(CH 3 ) 3 SiOSi(CH 3 ) 2 ] 2 O. This 2D INEPT DQF COSY experiment was then used to investigate the nature and the structure of the polymers which are formed in the early stages of the condensation reaction of diethoxydimethylsilane (DEDMS) in a DEDMS-water mixture containing a small amount of water at low pH(1.0)

NMR Study of isolobal N‐CH3+, N‐BH3 and N‐BF3 imidazole derivatives

Abstract: Some N-BH3, N-BF3, N+-CH3 and N+-H imidazole derivatives have been prepared and their 13C, 11B and 1H NMR spectra studied. The effect of the different substituents was analysed. The isolobal character of the N-BH3 and N-CH3 derivatives is reflected in their spectra. In the 13CH coupled spectra of N-borane adducts, the hydrides appear to be coupled with the heterocyclic carbon atoms. The C-4 chemical shift value is indicative of the nature of the N-substitution: lone pair (128–130 ppm), BH3 (126–128 ppm), BF3 and [CH3]+ (122–124 ppm) or [H]+ (117–119 ppm). The latter values allowed the chemical shifts of frozen imidazole to be estimated: 128 ppm for C-4 and 117 for C-5.

77Se, 13C and 1H NMR spectra of acyclic phenylselenenylalkanes and ‐alkenes and some of their selenoxides

Abstract: The 77 Se, 13 C and 1 H NMR spectra of 57 mostly new compounds with phenylselenenyl groups attached to acyclic alkanes, alkenes and phenylalkanes and of 10 corresponding selenoxides are reported. Substituent effects on the chemical shifts of these nuclei were determined and are discussed in terms of the conformation and configuration. The results suggest that, in contrast to PhSe-C-C-SePh fragments, C-C-C-Se chains prefer a gauche conformation

Studies of the π‐electron distribution in push–pull alkenes by 1H and 13C NMR spectroscopy—II

Abstract: A wide variety of push–pull alkenes were studied by means of variable-temperature 1H and 13C NMR spectroscopy with respect to the configuration/conformation and the barriers to rotation about partial CC and CN double bonds. For the assignment of the 13C NMR spectra especially the semi-selective INEPT pulse sequence and an incremental system for estimating the 13C chemical shift values of aromatic carbon atoms proved useful. The influence of thioether, sulphone and sulphoxide moieties in the acceptor part of the push–pull system on the π-electron distribution is critically considered.

Through-Space hydrogen-fluorine, carbon-fluorine and fluorine-fluorine spin-spin coupling in 2-phenyl-3-alkyl-4,5,6,7-tetrahydroindazoles

Abstract: 1H, 13C and 19F NMR experiments for a series of 3-alkyl-2-phenyl-4,5,6,7-tetrahydroindazoles revealed a six-bond through-space coupling between the ortho-fluorine and the hydrogen or fluorine atom of the position 3-alkyl group. This was further supported by NOE experiments. Molecular mechanics calculations on a representative structure indicated that several low energy conformers met the fluorine-carbon distance constraint suggested by the NMR data, and dynamic annealing experiments produced a conformer which was in complete agreement with the NMR data. This through-space interaction is speculated to be a result of repulsion between N-1 of the tetrahydroindazole and the ortho-fluorine lone pair electrons.

NMR spectroscopy of paramagnetic complexes. Part 39—natural abundance 2H NMR of paramagnetic sandwich compounds

Abstract: The methylated metallocenes (MeCp)2M with M = V, Cr, Mn, Co, Ni (1–5) were investigated by 1H and 2H NMR spectroscopy at natural abundance. The 2H NMR signals are narrower by a factor of up to 30 compared with the corresponding 1H NMR signals, thus establishing an inexpensive method and a general improvement of the NMR spectra of paramagnetic π-complexes. This has allowed the resolution of the signal splitting of Cp deuterons of 1 and 5 which could not be observed earlier in the 1H NMR spectra. The origin of the small (1 and 5) and large (2–4) signal splittings is discussed and related to an extended Huckel calculation. The relative magnitude of the signal splitting is reproduced, but the signal assignment had to be deduced from 13C NMR results. Primary isotope shifts of up to 4.0 ppm were found for 1, 2, 4 and 5. The much higher values for 3 (up to 16.6 ppm) reflect the combined influence of the intrinsic isotope shift and the isotope effect on the spin crossover.

199Hg NMR of aqueous solutions of inorganic mercury salts. Chemical shifts of HgCl2-n with n=0-4

Abstract: 119Hg NMR chemical shifts of the stable mercury species HgCl with n = 0–4 were determined by a least-squares algorithm from 199Hg chemical shifts measured in 50 mM aqueous mercury solution as a function of the chlorine concentration at 25°C. The concentrations of the corresponding mercury species in the solutions necessary in this special procedures were calculated by using published equilibrium constants. The results relative to dimethyl-mercury are −2256 ppm (Hg2+), −1897 ppm (HgCl+), −1560 ppm (HgCl2), −1436 ppm (HgCl3−) and −1152 ppm (HgCl) with maximum errors of ±5 ppm.

NMR of anthocyanins : assignments and effects of exchanging aromatic protons

Abstract: Pelargonidin-3-O-β-glucopyranoside was isolated from an acidified methanolic extract of frozen strawberries, purified by several chromatographical techniques and subjected to homo- and heteronuclear proton and carbon NMR analysis, including selective decoupling, DQF-COSY, NOESY, SEFT, HMQC and HMBC. Several strategies for the unambiguous assignment of the aglycone H-6 and H-8 resonances are discussed. Secondary isotope effects and different deuterium exchange rates of the protons were found.

19F NMR properties of some trifluorovinyl‐ and pentafluoropropenyl‐substituted aromatics

Abstract: 19 F NMR chemical shifts and coupling constants are given for a series of aryl (phenyl, naphthyl and phenanthryl) substituted trifluorovinyls and (E)- and (Z)-perfluoropropenyls. The relationship between the 19 F NMR chemical shifts and the configurational orientation of the perfluoroalkenyl substituents is discussed

Quantitative silicon‐29 NMR investigations of highly concentrated high‐ratio sodium silicate solutions

Abstract: In concentrated alkaline sodium silicate solutions a consistent decrease in the amount of silicate monomer Q0, end-groups Q1, cyclic trimer Q groups and middle groups Q2, and also branching groups in ring formations Q, is observed with increasing molar ratio of SiO2 to Na2O (RM), whereas for branching groups Q3 and cross-linking groups Q4 a consistent increase is found. The interest of this work is particularly focused on the Q4 region, which may indicate the formation of colloidal silicate particles. For this region the 29Si background signal from the NMR probe and the glass tube can cause problems, so the subtraction technique was used. Spin-lattice relaxation times, not previously reported for silicate solutions of concentrations and molar ratios as high as those studied here, are given for all structural units, including Q4. It was found that sodium silicate solutions of RM = 2.0 do contain a Q4 region when they are extremely highly concentrated in SiO2 (38 wt% SiO2) or aged.

Structure of oxidatively damaged nucleic acid adducts: PH dependence of the 13C NMR spectra of 8‐oxoguanosine and 8‐oxoadenosine

Abstract: 13 C NMR spectra of two C-8-oxo purine nucleosides, 8-oxoguanosine and 8-oxoadenosine, were recorded as a function of pH. The pH dependence of 13 C chemical shifts and 13 C- 1 H coupling constants associated with the purine carbon resonances was analyzed to probe the sites and extent of ionization and protonation of these oxidatively damaged nucleic acid adducts. In the course of this study, it was found that the C-2' and C-3' resonances of the ribofuranosyl sugar ring can be conveniently assigned on the basis of their unique pH-independent 1 H-coupling spectral patterns in aqueous solution

Two-dimensional 1H NMR studies on Desulfovibrio gigas ferredoxins. Assignment of the iron-sulfur cluster cysteinyl ligand protons

Abstract: 1D and 2D 1H NMR studies are reported on the oxidized and reduced [4Fe-4S] cluster of Desulfovibrio gigas ferredoxin I (Fdl). Several low-field contact shifted resonances (fast relaxing) are assigned to β-CH2 and α-CH coordinated cysteinyl residues. NOESY patterns (supported by 1D NOE experiments) resolves four pairs of geminal β-CH2 protons at low-field. The cluster ligands are assigned non-specifically to Cys8, Cys11, Cys14 and Cys50, based on the X-ray structural analysis available for the oligomeric form, FdII, that contains a single [3Fe-4S] cluster. It was indicated in this case that Cys11 is not bound to the trinuclear cluster but is tilted towards the solvent. The presence of four pairs of geminal β-CH2 protons for FdI unambiguously proves the occupancy of the fourth site of the [3Fe-4S] complex and implies the coordination of the Cys11 at the cluster. Analysis of the oxidized form of FdII, using the same methodology as described for FdI, supports the presence of three cysteinyl ligands in the [3Fe-4S] core. Further, the combined use of the X-ray coordinates enables the specific assignment of the three cysteinyl ligands of the cluster, extending a previous assignment of Cys50. In addition, very broad resonances were detected for the reduced form of FdII in the low-field region around 200 ppm and in the high field region around −80 ppm.

Chemical shift references for 77Se NMR spectroscopy. Selenous acid

Abstract: The temperature and concentration dependences of the chemical shift of aqueous selenous acid were studied in order to establish a precise relationship between the shift of this convenient reference and that of dimethyl selenide. The chemical shift of saturated H2SeO3 in H2O on the Me2Se shift scale at 20°C is 1300.0 ppm and its temperature coefficient is 0.094 ppm K−1. The relationships to the chemical shifts of Me2Se in CDCl3 and SeO2 in D2O are also discussed. The results permit the correlation of 77Se NMR shifts from several sources.